Defeat resistant burglar alarm contact

ABSTRACT

The present invention comprises a defeat resistant burglar alarm contact of miniature size for application on small safes having rotating doors as well as for the protection of other types of doors and closure elements. Special features have been provided to resist various forms of burglarious attack upon the protected closure element.

ted States Patent Byrne Nov. 6, 1973 [54] DEFEAT RESISTANT BURGLAR ALARM3,444,493 5/1969 Nieuwhof 335/206 CONTACT 2,912,540 11/1959 Sawicki340/274 UX 3,510,813 5/1970 Bozzone 335/206 X Inventofl Thomas J- y NewYork, NY 3,544,934 12/1970 Poliakoff 335/206 Assigneez American DisrictTelegraph 2,877,361 3/1959 Chase 340/274 X Company, Jersey City, NJ.Primary Examiner-John W. Caldwell 2] Filed! 1972 AssistantExaminer-William M. Wannisky 2'1 Appl. No.: 232,871 Att0rney-Ronald F.Ball 52 us. c1 340/274, 340/276, 335/219, [57] ABSTRACT 200/6153 Thepresent invention comprises a defeat resistant bur- [51] Int. Cl. 6081113/08 g alarm Contact of m n atur s z f r application on [58] Field ofSearch 340 274, 276; small Safes having rotating doors as Well as forthe P 335/206, 205, 219; 200/6158; 315/34 tection of other types ofdoors and closure elements. Special features have been provided toresist various 56 R i Ci L forms of burglarious attack upon theprotected closure UNITED STATES PATENTS element 3,161,742 12/1964 Bagno335/206 X 9 Claims, 6 Drawing Figures DEFEAT RESISTANT BURGLAR ALARMCONTACT BACKGROUND OF THE INVENTION Burglar alarm contacts have beenused with notable success for many years by the electrical protectionindustry to signal the unauthorized opening of doors, windows and othertypes of closure elements designed to protect property against theft.

The earliest types of contacts were simple mechanical devices capable ofopening and/or closing electrical circuits. Shortcomings lay in theirrelatively large size and more particularly in the troubles resultingfrom corrosion of current carrying metal surfaces which dften causedburglar alarm systems to become inoperative at the most inconvenienttimes. The fairly recent advent of miniature switches hermeticallysealed in glass envelopes and operated by the approach or withdrawal ofa magnet has largely overcome the size and corrosion problems.

A particular problem remained, however, in the case of closure elementsof small size as exemplified by safes having round openings with adiameter on the order of six inches. Physical size limitations make itimpossible to install most of the mechanical types of contact on suchsafes. Another serious problem arises when attempts are made to utilizethe small size magnetic type of contact for'such installations in thatthe magnetic contact in its simple form might be compromised by aknowledgeable burglar who may then proceed to loot the safe with no fearof a signal being transmitted by the burglar alarm system.

Past efforts to improve the defeat resistance of the magnetic type ofburglar alarm contact are exemplified by United States Pat. No.2,877,361 issued to L.H. Chase on Mar. 10, 1959, US. Pat. No. 2,912,540issued to A.T. Sawicki on Nov. 10, 1959 and US. Pat. No. 3,187,129issued to 1.131. McBrian on June 1, 1965. These patents all require theuse of a second or biasing magnet which is employed to establish abalanced magnetic field and experience has shown that such arrangementsare difficult to maintain in proper adjustment under service conditions.Still other forms of magnetically responsive switches for use in burglaralarm systems are shown in US. Pat. No. 3,621,417 issued to J.F. Halabyon Nov. l6, 1971.

Accordingly, the principal object of the present invention has been theprovision of a defeat resistant burglar alarm contact. Experience hasshown that it is foolhardy to refer to any protection device or systemas being completely defeatproof, but the preferred contact constructionof the present invention has been subjected to repeated. defeat attemptsby highlytrained engineers thoroughly familiar with burglar alarmsystems and has successfully resisted all such attempts.

Another object has been the provision of a burglar alarm contact of suchreduced overall dimensions as to permit installation upon the rounddoors of small safes.

A still further object has been the provision of a burglar alarm contactthat is simple in principle and that does not require difficult fieldadjustments.

Other and further objects, features and advantages of the invention willbe apparent from the following description of the invention.

BRIEF DESCRIPTION OF THE INVENTION The invention comprises thecombination of a contact block and an actuating block. The contact blockis mounted on the stationary frame of the door to be protected andcontains two magnetically operable transfer switches connected in serieswith a conventional burglar alarm system. One of theswitches ispartially shielded by a plate of a high permeability metal to providedefeat resistance properties. The actuating block is mounted on the doorand comprises a magnet whose withdrawal as the door is opened operatesthe contact block to provide an electrical signal in the burglar alarmsystem. In accordance with a particular feature of the invention, thefield of the magnet is partially shunted by the steel door of the safeto provide resistance to a particular form of attack.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described ingreater detail with reference to the appended drawings in which:

FIG. 1 is a front view showing the manner of installation of theinvention upon a safe having a round openmg;

FIG. 2 is a fragmentary vertical sectional view taken generally alongthe line 22 of FIG. 1;

FIG. 3 is a plan view in section of the actuating block assembly takenalong the line 33 of FIG. 2;

FIG. 4 is a plan view in section of the contact block assembly takengenerally along the line 44 of FIG. 2;

FIG. 5' is a plan view looking upward, with parts broken away, of thecontact block assembly, taken along the line 5-5 of FIG. 2; and a FIG. 6is a schematic electrical diagram showing the inclusion of thepresentinvention in a burglar alarm system.

DESCRIPTION OF THE PREFERRED EMBODIMENT Those skilled in the art willrecognize that the burglar alarm contact of the present invention may beapplied with equal facility to a wide variety of closure elements andthat the following description thereof in connection with a roundopeningsafe is by way of illustration only and is not intended as a limitationupon the use of the invention.

Referring now to FIG. 1, 10 indicates a round opening safe such as theModel OOX manufactured by the Mosler Safe Company. The safe is 11 incheshigh, 11 inches wide and has a door opening diameter of 6 1% inches.When the proper combination has been dialed on knob 11, handle 12 andthe rotatable cylinder portion 13 of the door 14 are rotated about 30degrees and the entire door 14'swung forward on hinges 15 to reveal around opening to the interior of the safe.

Actuating block 16 is mounted on the exposed or outside of rotatingportion 13 of the safe door 14 and contact block 17 is mounted on theoutside of door 14 by conventional screw fasteners as will be describedin more detail hereinafter. Cable 18 connects the contact block 17 to aconventional burglar alarm system.

The contact and actuating blocks are identical in appearance (save forthe cable connection) and are approximately inch deep, 2 inches wide andl 14; inches high. Blocks l6 and 17 are not sensitive to orientation inspace and may be mounted at any point around the periphery of rotatingportion 13 of the door as may be most convenient in the particularinstallation. It is only necessary to allow operating clearance for thehandle 12 and to mount actuating block 16 entirely within the outerdiameter of the rotating portion 13 of the door so that block 16 willclear block 17 when portion 13 is rotated.

As best shown in FIGS. 2 and 3, actuating block 16 comprises a cover 19,a magnet 20, a bracket 21 and a rectangularly shaped bar 22. Bracket 21,which may be conveniently made of light gauge aluminum stock, is securedto the rotating portion 13 of the safe by means of two screws 23. Sinceit is common practice to case harden safes to protect them againstburglarious attack, it may not be possible to drill the holes for thescrews 23. In such event, a rectangular piece of soft steel (not shown)which may be drilled and tapped is welded to rotating portion 13 toserve as a mount for the-actuating block 16.

Bracket 21 has attached thereto by means of screws 24 a rectangularlyshaped bar 22. Bar 22 may be of aluminum stock and has a bore 25 drilledtherein for supporting cylindrical bar magnet which is sealed therein bymeans of an epoxy cement or similar adhesive. Bar magnet 20 may be madefrom a permanent magnet alloy such as Alnico V which has a compositionof aluminum 8 percent, nickel 14 percent, cobalt 24 percent, copper 3percent, and the balance iron. The cover 19, which may be convenientlydrawn from thin gauge aluminum stock, is secured to the bar 22 by twoflat headed screws 26.

As best shown in FIGS. 2, 4 and 5, contact block 17 comprises a cover27, a contact assembly generally indicated as 28, a magnetic shield 29and a plastic electrical insulating shield-30. The contact assembly 28consists of a panel 31 for supporting the electrical components, a base32 and a mounting block 33. Panel 31 and block 33 are conveniently madefrom a phenolic composition or other nonconducting material. Base 32 maybe made of light gauge aluminum stock. The panel 31 is fastened to block33 by two screws 34 and the base 32 is fastened to block 33 by'two flathead screws 35. Assembly 28 is secured to door 14 by two screws 36. Ifthe safe has been hardened and a soft steel plate has been weldedthereto to mount the actuating block 16, as previously described, asimilar plate of the same thickness should be employed to mount contactblock In the embodiment shown the contact block 17 is mounted on safedoor 14 so that the contact block is adjacent the actuating block 16mounted on the rotating portion 13 of the door when the rotating portionof the door is in its normal closed and locked position, as shown inFIG. 1. It will be understood by those skilled in the art that thepositions of the contact block 16 and the actuating block 17 may bereversed. It will be further understood that where the burglar alarmcontact is used for other forms of openings, such as building doors,windows and the like, either the contact block or the actuating block ismounted on a fixed part of the door opening and the remaining actuatingor contact block, respectively, is mounted on a movable portion of theopening so as to be adjacent the other portion of the block when theopening is in a closed position.

Two magnetically operated transfer switches SW1 and SW2 of the glassenclosed type are mounted bysix conventional terminal posts 37 on panel31 and connected by means of a four wire cable 18 to the associatedexternal burglar alarm circuit, as will be explained in conjunction withFIG. 6.

Contact block cover 27 is secured to mounting block 33 with two flathead screws 38 and is generally similar to actuating block cover 19except for the cable attachment 18 and the magnetic shield 29 secured tocover 27 in such position as to partially shield switch SW2 from magnet20.

Shield 29 may be made from a thin sheet of metal having a high magneticpermeability, such as a nickel-copper-chromium-iron alloy sold under thetrademark ,Mumetal of the Allegheny Ludlum Corporation. This particularalloy has a typical composition of 77.2 percent nickel, 4.8 percentcopper, 1.5 percent chromium, and the balance iron. Other materials maybe used provided they have the desired characteristic of high magneticpermeability. Shield 29 may be secured to cover 27 with an epoxy cementor other suitable adhesive.

Insulating shield 30, which may be conveniently made of a plastic havinggood electrical insulating properties such as polyethylene terephthalatesold under the trademark Mylar polyester film by E.l. du Pont de Nemours& Co., is inserted within cover 27. Its purpose is to prevent accidentalshorting of the terminals 37 by the metallic cover 27. Such shortingwould be likely to occur if the cover were dented or otherwise deformed.The device will function normally without the plastic insulator as longas the cover is in its proper position. As will be appreciated by thoseskilled in the art, it is important to guard against accidental shortingbecause a false alarm could be produced thereby, resulting inunnecessary dispatching of the police and other undesirable activity. Innormal practice a plastic insulator of the type described will beprovided when the metallic cover 27 has little rigidity or when there isa possibility of shorting terminals 37 when removing the cover 27 whilethe current is on.

The burglar alarm contact of the invention may be connected to aconventional burglar alarm system of the central station type as shownin a simplified-form by FIG. 6. A central station 39, which is manned atall times by operators trained .to take appropriate action in responseto incoming signals, is connected to a protected premise 40 by atransmission line 41. The line 41, which is ordinarily a telephone lineleased for the purpose, may be up to approximately 20 miles in lengthand may pass through several telephone exchanges (not shown).

The system" is powered by battery 42 at the central station 39 which maysupply a voltage in the range of from 52 to 78 volts. The negativeterminal is connected to ground at 43 and the positive terminal isconnected to the transmission line 41 through a break drop 44 and aground drop 45.

The break drop 44 is a current sensitive device which responds to a dropin the line current below a predetermined value while the ground drop 45is a similar device which responds to an increase in line current aboveanother predetermined value. Such devices are well known in the art inthe form of relays and as solid state electronic devices. In eitherform, operation of the devices will result in audible and visual signalsto attract the attention of the operator.

Each of the spaced, series connected magnetically actuable switchelements SW1 and SW2 has a front (48, 54 respectively) and back (53, 50respectively) electrical contact and an armature (47, 49 respectively)movable between the front and back contacts.

Each of the armatures is biased so that in the absence of a magneticfield the armature is in engagement with the back contact. Subjectingeither switch to the etfect of a magnetic field will cause the armatureto move from the back contact to the front contact.

Switch SW1 and magnetic shield 29 within contact block 17 are locatedrelative to each other and to magnet 20 within actuating block 16 whenthe rotatable portion 13 of door 14 is in closed position (as shown inFIG. 1) so that switch SW1 is responsive to the magnetic field of magnet20. Switch SW2, on the other hand, is located relative to magneticshield 29 so as to be shielded from the magnetic field of magnet 20 bythe magnetic shield when the rotatable portion 13 of door 14 is inclosed position.

At the protected premises, the current flows from line 41 through acontrol set 46 of conventional design used to turn the protection systemOFF and ON, through armature 47 and front contact 48 of switch SW1 whichis held in the operative or front contact position by magnet 20. SwitchSW2, however, is in the unoperative or back contact position by'reasonof the magnetic shield 29. Thus the current path is via armature 49 andback contact 50 of switch SW2 to the other protection devices indicatedas 51 to ground 52 at the protected premises 40. Back contact 53 of theswitch- SW1 and front contact 54 of switch SW2 are connected togetherand to ground 52 by a conductor 55 which forms a shunt path.

Those skilled in the art will recognize that the, term other protectiondevices is intended to include other alarm contacts used to protectentrance doors and windows, foil patterns installed for the protectionof glass windows and doors plus such other intrusion detection devicesas floor traps, photoelectric beams and ultrasonic and microwave motiondetectors as are normally used for premises protection.

It will also be understood that the alarm contact of the invention neednot be. installed between the control set and the other protectiondevices as shown in FIG. 6. As is well known in the art, it is onlynecessary that there be enough resistance in the circuit so that theoperation of the switches SW1 andlor SW2 will shunt sufficientresistance out of the current path to allow the current flow to increasesufficiently I to operate the ground drop '45. Furthermore, while aground return has been shown between the protected premises and thecentral station, a line return could as well be emthan the centralstation type, such as those connected directly to a police headquartersor other signal receiving location and in local systems arranged tosound a bell or horn outside the protected premises upon the occurrenceof an alarm.

OPERATION OF THE SYSTEM FIG. 6 shows the burglar alann system in theprotection ON condition withcurrent flowing from battery 42 at centralstation 39 via transmission line 41 to protected premises 40 and thencethrough control set 46, contact block 17 and the other protectiondevices 51 to return to the central station 39 via ground connections52, 43.

If an attempt is now made to obtain access to the interior of safe 10via rotation of door portion l3, the actuating block 16 will be removedfrom its normal location opposite contact block 17. Switch SW1 is thusfreed from the field of magnet 20. Because, aspreviously explained,armature 47 of SW1 is normally biased to contact the back contact 53,armature 47 transfers from front contact 48 to back contact 53. The flowof current in the circuit is first interrupted as armature 47 leavescontact 48 which is detected and annunciated by the break drop 44 atcentral station 39. When armature 47 makes with back contact 53, aportion of the circuit is shunted via conductor 55 and the resultingincrease in current flow is detected and annunciated by the ground drop45 at central station 39.

Those skilled in the art will recognize that, while the magnet 20 wasremoved from the presence of switch SW1 by a rotational movement in theforegoing illustration, the direction of removal is not significant tothe operation of switch SW1. Accordingly, the alarm contact of theinvention may be employed on a wide variety of closure elements whichopen in various manners.

Should an attempt be made to render the protection system inoperative bysevering the cable 18, a break signal will be registered by the breakdrop 44 as the continuity of the circuit would thus be interrupted.Furthermore, in accordance with one of the features of the invention, anintruder cannot gain access to the interior of either actuating block 16or contact block 17 when the safe is in the closed condition because theU16 to 1/4 inch spacing between the blocks is not sufficient to permitaccess to the screws 26, 38 which secure the block covers 19, 27,respectively, to bar 22 and block 33, respectively. I

A well known form of attack which has been employed successfully againstalarm contacts of the magnetic type is to place an external magnetagainst the contact block in such manner that the field of the externalmagnet will hold the switch in the normal position while the actuatingblock is removed. The present invention is resistant against such attackby reason of the switch SW2. The introduction of an external magnet willcause switch- SW2 to operate and armature 49 thereof will transfer fromback contact 50 to front contact 54. As was the case when switch SW1 wasoperated, the circuit will first be opened when armature 49 leavescontact 50 and then grounded when armature 49 makes with contact 54thereby producing both break and ground signals at centralstation 39.

In accordance with a particularly important feature of the invention,resistance is provided against the type of attack in which the actuatingblock and the contact block are taped together and removed from theprotected safe as a unit. In the illustrated embodiment of the presentinvention the actuating block is mounted on the movable portion 13 ofthe safe opening, which portion 13 is of magnetic material, preferablysteel. The magnetic field of magnet 20 is partially shunted by themagnetic movable portion 13 of the door. The block mounting screws 23,36 are not accessible being inside the block covers 19 and 27,respectively. However, if it were possible in some way to detach blocks16 and 17 as a unit from the safe, an alarm would still be transmittedbecause the field of magnet 20 is partially shunted by the steel frameof the safe when the alarm contact is properly mounted. When themagnetic shunt is removed by the withdrawal of the alarm contact fromthe safe, the effective field 'of magnet 20 will increase sufficientlyto cause the armature 49 of the switch SW2 to move from back contact 50to front contact 54 thus originating break and ground signals at thecentral station as previously described.

A further resistance to attack against the integrity of the system isafforded because the magnetic shield 29 is secured to the cover 27 aspreviously described. Removal of the cover 27 will result insimultaneous withdrawal of the magnetic shield 29, thus subjectingswitch SW2 to the magnetic field of magnet 20. Armature 49 of switch SW2will transfer from back contact 50 to front contact 54 producing bothbreak and ground signals at the central station.

While the invention has been described in connection with specificembodiments thereof and in a specific use, various modifications thereofwill occur tothose skilled in the art without departing from the spiritand scope of the invention as set forth in the appended claims.

What is claimed is:

l. A burglar alarm contact for door openings and the like comprising acontact block and an actuating block, one of which is mounted on a fixedpart of the door opening and the other of which is mounted on a movableportion of said opening so as to be adjacent said one block when saidopening is in a closed position, a magnet, means mounting said magnetwithin said actuating block, said contact block comprising a magneticshield, spaced series connected first and second magnetically actuableswitch elements arranged to be connected to an external alarm circuit,said first of said switch elements being located relative to said magnetand said magnetic shield when said opening is in said closed position soas to be responsive to the magnetic field of said magnet, said second ofsaid switch elements being located so as to be shielded from themagnetic field of said magnet by said magnetic shield when said openingis in said closed position, said second of said switch elements beingunresponsive to the movement of said magnet relative to said magneticshield when said movable portion of said door opening is moved out ofclosed'position, each of said switch elements having a'front and backelectrical contact and an armature movable between said front and backcontacts, and alarm circuit means coupled to said switch elements andarranged to transmit an alarm signal when either said armature of saidfirst switch element contacts the back contact thereof or when saidarmature of said second switch element contacts the front contactthereof.

2. A burglar alarm contact for door openings and the like comprising acontact block and an actuating block, one of which is mounted on onepart of the door opening and the other of which is mounted on a secondpart of the door opening, said parts of said door opening being movablerelative to each other between first and second positions, said contactand actuating blocks being mounted so as to be adjacent each other whensaid parts of said door opening are in said first position, a magnet,-meansmounting said magnet within said actuating block, said contactblock comprising a magnetic shield, spaced series connected first andsecond magnetically actuable switch elements arranged to be connected toan external alarm circuit, said first of said switch elements beinglocated relative to said magnet and said magnetic shield when said partsof said door opening are in said first position so as to be responsiveto the magnetic field of said magnet, said second of said switchelements being located so as to be shielded from the magnetic field ofsaid magnet by said magnetic shield when said parts of said door openingare in said first position, said second of said switch elements beingunresponsive to the movement of said magnet relative to said magneticshield when said parts of said door opening are moved out of said firstposition, each of said switch elements having a front and backelectrical contact and an armature movable between said front and backcontacts, and alarm circuit means coupled to said switch elements andarranged to transmit an alarm signal when either said armature of saidfirst switch element contacts the back contact thereof or when saidarmature of said second switch element contacts the front contactthereof.

3. A burglar alarm contadt for door openings and the like comprising acontact block and an actuating block, one of which is mounted on a fixedpart of the door opening and the other of which is mounted on a movableportion of said opening so as to be adjacent said one block when saidopening is in a closed position, a magnet, means mounting said magnetwithin said actuating block, said contact block comprising a magneticshield, spaced series connected first and second magnetically actuableswitch elements arranged to be connected to an external alarm circuit,said first of said switch elements being located relative to said magnetand said magnetic shield when said opening is in said closed position soas to be responsive to the magnetic field of said magnet, said second ofsaid switch elements being located so as to be shielded from themagnetic field of said magnet by said magnetic shield when said openingis in said closed position, said second of said switch elements beingunresponsive to the movement of said magnet relative to said magneticshield when said movable portion of said door opening is moved out ofclosed position, each of said switch elements having a front and backelectrical contact and an armature movable between said front and backcontacts, the armature of said first switch element engaging said frontcontact when said opening is in said closed position and the armature ofsaid second switch element engaging said back contact when said openingis in said closed position, and alarm circuit means coupled to saidswitch elements and arranged to transmit an alarm signal ien either saidarmature of said first switch element is removed from contact with saidfront contact thereof or when said armature of said second switchelement is removed from contact with said back contact thereof.

4. A burglar alarm contact according to claim 3 wherein said magneticshield comprises a thin sheet of metal having a high magneticpermeability.

5. A burglar alarm contact according to claim 4 wherein said magneticshield comprises a nickel-copper-chromium-iron alloy.

6. A burglar alarm contact for metallic door openings and the likecomprising a contact block mounted on a fixed part of the door openingand an actuating block mounted on a movable portion of said opening soas to be adjacent said contact element when said opening is in a closedposition, said movable portion of said opening being of magneticmaterial, a magnet, means mounting said magnet within said actuatingblock, said contact block comprising a magnetic shield, spaced seriesconnected first and second magnetically actuable switch elementsarranged to be connected to an external alarm circuit, said first ofsaid switch elements being located relative to said magnet and saidmagnetic shield when said opening is in said closed position so as to beresponsive to the magnetic field of said magnet, said second of saidswitch elements being located so as to be shielded from the magneticfield of said magnet by said magnetic shield when said opening is insaid closed position, said second of said switch elements beingunresponsive to the movement of said magnet relative to said magneticshield when said movable portion of said door opening is moved out ofclosed position, each of said switch elements having a front and backelectrical contact and an armature movable between said front and backcontacts, the armature of said first switch element engaging said frontcontact when said opening is in said closed position and engaging saidback contact when the effect of the magnetic field from said magnet isremoved from said first switch element, the armature of said second.switch element engaging said back contact thereof when said opening isin said closed position and engaging said front contact thereof wheninfluenced by a magnetic field other than from said magnet, and alarmcircuit means coupled to said switch elements and arranged to transmitan alarm signal when either said armature of said first switch elementmoves from the front to the back contact thereof or when said armatureof said second switch element moves from the back to the front contactthereof, the field of said magnet being partially shunted by themagnetic movable portion of said opening whereby removal of saidmagnetic shunt will increase the effective field of said magnetsufficiently to cause said armature of said second switch element tomove from said back to said front contact thereof.

7. A burglar alarm contact for rotatable metallic door openingscomprising a contact block mounted on one part of said door opening andan actuating block mounted on a second magnetic part of said dooropening, said parts of said door opening being movable relative to eachother between first and second positions, said contact and actuatingblocks being mounted so as to be adjacent each other when said parts ofsaid door opening are in said first position, a magnet, means field ofsaid magnet, said second of said switch elements being located so as tobe shielded from the magnetic field of said magnet by said magneticshield when said parts of said door opening are in said first position,said second of said switch elements being unresponsive to the movementof said magnet relative to said magnetic shield when said parts of saiddoor opening are moved out of said first position, each of said switchelements having a front and back electrical contact and an armaturemovable between said front and back contacts, the armature of said firstswitch element engaging said front contact when said parts of said dooropening are in said first position and engaging said back contact whenthe effect of the magnetic field from said magnet is removed from saidfirst switch element, the armature of said second switch elementengaging said back contact thereof when said parts of said door openingare in said first position and engaging said front contact thereof wheninfluenced by a magnetic field other than from said magnet, and alarmcircuit means coupled to said switch elements and arranged to transmitan alarm signal when either said armature of said first switch elementmoves from the front to the back contact thereof or when said armatureof said second switch element moves from the back to the front contactthereof, the field of said magnet being partially shunted by themagnetic movable portion of said opening whereby removal of saidmagnetic shunt will increase the effective field of said magnetsufficiently to cause said armature of said second switch element tomove from said back to said front contact thereof.

8. A burglar alarm contact according to claim 7 wherein said magneticshield comprises a thin sheet of metal having a high magneticpermeability.

9. A burglar alarm contact according to claim 8 wherein said magneticshield comprises a nickel-copper-chromium-iron alloy.

1. A burglar alarm contact for door openings and the like comprising acontact block and an actuating block, one of which is mounted on a fixedpart of the door opening and the other of which is mounted on a movableportion of said opening so as to be adjacent said one block when saidopening is in a closed position, a magnet, means mounting said magnetwithin said actuating block, said contact block comprising a magneticshield, spaced series connected first and second magnetically actuableswitch elements arranged to be connected to an external alarm circuit,said first of said switch elements being located relative to said magnetand said magnetic shield when said opening is in said closed position soas to be responsive to the magnetic field of said magnet, said second ofsaid switch elements being located so as to be shielded from themagnetic field of said magnet by said magnetic shield when said openingis in said closed position, said second of said switch elements beingunresponsive to the movement of said magnet relative to said magneticshield when said movable portion of said door opening is moved out ofclosed position, each of said switch elements having a front and backelectrical contact and an armature movable between said front and backcontacts, and alarm circuit means coupled to said switch elements andarranged to transmit an alarm signal when either said armature of saidfirst switch element contacts the back contact thereof or when saidarmature of said second switch element contacts the front contactthereof.
 2. A burglar alarm contact for door openings and the likecomprising a contact block and an actuating block, one of which ismounted on one part of the door opening and the other of which ismounted on a second part of the door opening, said parts of said dooropening being movable relative to each other between first and secondpositions, said contact and actuating blocks being mounted so as to beadjacent each other when said parts of said door opening are in saidfirst position, a magnet, means mounting said magnet within saidactuating block, said contact block comprising a magnetic shield, spacedseries connected first and second magnetically actuable switch elementsarranged to be connected to an external alarm circuit, said first ofsaid switch elements being located relative to said magnet and saidmagnetic shield when said parts of said door opening are in said firstposition so as to be responsive to the magnetic field of said magnet,said second of said switch elements being located so as to be shieldedfrom the magnetic field of said magnet by said magnetic shield when saidparts of said door opening are in said first position, said second ofsaid switch elements being unresponsive to the movement of said magnetrelative to said magnetic shield when said parts of said door openingare moved out of said first position, each of said switch elementshaving a front and back electrical contact and an armature movablebetween said front and back contacts, and alarm circuit means coupled tosaid switch elements and arranged to transmit an alarm signal wheneither said armature of said first switch element contacts the backcontact thereof or when said armature of said second switch elementcontacts the front contact thereof.
 3. A burglar alarm contact for dooropenings and the like comprising a contact block and an actuating block,one of which is mounted on a fixed part of the door opening and theother of which is mounted on a movable portion of said opening so as tobe adjacent said one block when said opening is in a closed position, amagnet, means mounting said magnet within said actuating block, saidcontact block comprising a magnetic shield, spaced series connectedfirst and second magnetically actuable switch elements arranged to beconnected to an external alarm circuit, said first of said switchelements being located relative to said magnet and said magnetic shieldwhen said opening is in said closed position so as to be responsive tothe magnetic field of said magnet, said second of said switch elementsbeing located so as to be shielded from the magnetic field of saidmagnet by said magnetic shield when said opening is in said closedposition, said second of said switch elements being unresponsive to themovement of said magnet relative to said magnetic shield when saidmovable portion of said door opening is moved out of closed position,each of said switch elements having a front and back electrical contactand an armature movable between said front and back contacts, thearmature of said first switch element engaging said front contact whensaid opening is in said closed position and the armature of said secondswitch element engaging said back contact when said opening is in saidclosed position, and alarm circuit means coupled to said switch elementsand arranged to transmit an alarm signal when either said armature ofsaid first switch element is removed from contact with said frontcontact thereof or when said armature of said second switch element isremoved from contact with said back contact thereof.
 4. A burglar alarmcontact according to claim 3 wherein said magnetic shield comprises athin sheet of metal having a high magnetic permeability.
 5. A burglaralarm contact according to claim 4 wherein said magnetic shieldcomprises a nickel-copper-chromium-iron alloy.
 6. A burglar alarmcontact for metallic door openings And the like comprising a contactblock mounted on a fixed part of the door opening and an actuating blockmounted on a movable portion of said opening so as to be adjacent saidcontact element when said opening is in a closed position, said movableportion of said opening being of magnetic material, a magnet, meansmounting said magnet within said actuating block, said contact blockcomprising a magnetic shield, spaced series connected first and secondmagnetically actuable switch elements arranged to be connected to anexternal alarm circuit, said first of said switch elements being locatedrelative to said magnet and said magnetic shield when said opening is insaid closed position so as to be responsive to the magnetic field ofsaid magnet, said second of said switch elements being located so as tobe shielded from the magnetic field of said magnet by said magneticshield when said opening is in said closed position, said second of saidswitch elements being unresponsive to the movement of said magnetrelative to said magnetic shield when said movable portion of said dooropening is moved out of closed position, each of said switch elementshaving a front and back electrical contact and an armature movablebetween said front and back contacts, the armature of said first switchelement engaging said front contact when said opening is in said closedposition and engaging said back contact when the effect of the magneticfield from said magnet is removed from said first switch element, thearmature of said second switch element engaging said back contactthereof when said opening is in said closed position and engaging saidfront contact thereof when influenced by a magnetic field other thanfrom said magnet, and alarm circuit means coupled to said switchelements and arranged to transmit an alarm signal when either saidarmature of said first switch element moves from the front to the backcontact thereof or when said armature of said second switch elementmoves from the back to the front contact thereof, the field of saidmagnet being partially shunted by the magnetic movable portion of saidopening whereby removal of said magnetic shunt will increase theeffective field of said magnet sufficiently to cause said armature ofsaid second switch element to move from said back to said front contactthereof.
 7. A burglar alarm contact for rotatable metallic door openingscomprising a contact block mounted on one part of said door opening andan actuating block mounted on a second magnetic part of said dooropening, said parts of said door opening being movable relative to eachother between first and second positions, said contact and actuatingblocks being mounted so as to be adjacent each other when said parts ofsaid door opening are in said first position, a magnet, means mountingsaid magnet within said actuating block, said contact block comprising amagnetic shield, spaced series connected first and second magneticallyactuable switch elements arranged to be connected to an external alarmcircuit, said first of said switch elements being located relative tosaid magnet and said magnetic shield when said parts of said dooropening are in said first position so as to be responsive to themagnetic field of said magnet, said second of said switch elements beinglocated so as to be shielded from the magnetic field of said magnet bysaid magnetic shield when said parts of said door opening are in saidfirst position, said second of said switch elements being unresponsiveto the movement of said magnet relative to said magnetic shield whensaid parts of said door opening are moved out of said first position,each of said switch elements having a front and back electrical contactand an armature movable between said front and back contacts, thearmature of said first switch element engaging said front contact whensaid parts of said door opening are in said first position and engagingsaid back contact when the effect of the magnetic field from said magnetis removed from said firsT switch element, the armature of said secondswitch element engaging said back contact thereof when said parts ofsaid door opening are in said first position and engaging said frontcontact thereof when influenced by a magnetic field other than from saidmagnet, and alarm circuit means coupled to said switch elements andarranged to transmit an alarm signal when either said armature of saidfirst switch element moves from the front to the back contact thereof orwhen said armature of said second switch element moves from the back tothe front contact thereof, the field of said magnet being partiallyshunted by the magnetic movable portion of said opening whereby removalof said magnetic shunt will increase the effective field of said magnetsufficiently to cause said armature of said second switch element tomove from said back to said front contact thereof.
 8. A burglar alarmcontact according to claim 7 wherein said magnetic shield comprises athin sheet of metal having a high magnetic permeability.
 9. A burglaralarm contact according to claim 8 wherein said magnetic shieldcomprises a nickel-copper-chromium-iron alloy.